1. Field of the Invention
The present invention relates generally to the asexual reproduction of higher plants, and more particularly to a method for acclimatizing plant propagules obtained by in vitro tissue culture methods and for planting the acclimatized propagules prior to root formation in the field.
Plant tissue culture refers to various methods which have been developed for asexually reproducing plants from juvenile and rejuvenated mature plant material. Typically, a shoot, shoot tip, bud, stem section, or stem base is excised from a growing plant, disinfested, and placed in appropriate hormone and nutrient solutions to cause auxillary growth and shoot lengthening in a multiplication culture to form multiple plant propagules. The advantage of such asexual reproduction is that mature plants having desirable characteristics can be replicated precisely if appropriate steps are taken.
Various methods are presently employed for the rooting and acclimatization of such tissue culture propagules. For example, rooted shoots from multiplication cultures may be transferred to individual containers having an appropriate potting mix and maintained in a greenhouse to allow gradual acclimatization. Humidity levels in the greenhouse are initially elevated but gradually reduced as the plantlet becomes acclimatized to the new environment. After sufficient acclimatization has occurred, the plantlet may be transferred to the field with a reasonable chance of survival. This approach, although workable for many plants, suffers from the need to employ the intermediate greenhouse acclimatization step which may last from 4 to 6 weeks, or longer. Such acclimatization in the greenhouse greatly increases the expense of producing the field plantlets, but heretofore has been considered necessary for the survival of many types of plants.
Field acclimatization of many plants has proved problematic even when great care is taken in making the transition from tissue culture to the field. For example, many woody tree species suffer from root acclimatization problems even when care is taken to acclimatize the shoot portion of the plantlet to ambient field conditions prior to planting.
For the above reasons, it would be desirable to provide an improved method for root induction and acclimatization of plant propagules obtained from tissue culture. It would be particularly desirable to provide such a method which would allow the direct field planting of such plant propagules prior to root formation to facilitate root acclimatization of the resulting plantlet.
2. Description of the Prior Art
The propagation of Paradox walnut root stock by shoot tip tissue culture is described in Driver and Kuniyuki (1984) Hort.Science 19:507-509. The method relies on multiple shoot formation resulting from the tissue culture of nodal explants in a suitable basal medium, followed by shoot formation in the presence of N6-benzyladenine and indole-3-butyric acid. An optimum medium for inducing multiple shoot formation of Paradox walnut is described and designated DKW. Root formation is induced in the presence of indole-3-butyric acid and naphthaleneacetic acid. Plantlets were grown from root shoots in a synthetic potting mix in a misting chamber over a 6 week period. Such plantlets were found able to withstand uncontrolled environmental conditions in small-scale trials.
The micropropagation of fruit trees and plants is discussed in the following references: Broome and Zimmerman, "Culture of Shoot Meristems: Fruit Plants," in Cell Culture and Somatic Cell Genetics of Plants, Vol. 1, Academic Press, Inc., New York, pp. 111-122; Dunstan and Turner, "The Acclimatization of Micropropagated Plants," in Cell Culture and Somatic Cell Genetics of Plants, Vol. 1, supra, pp. 123-129; Fuchigami et al. (1981) J. Amer. Soc. Hort. Sci. 106:519-522; and Brainerd and Fuchigami (1981) J. Amer. Soc. Hort. Sci. 106:515-518. See also Tissue Culture and Forestry, Bonga and Durzan, eds., Martinus Hijholf, Netherlands (1982), for a discussion of the micropropagation of other woody plants.